We tend to think of planets and stars as very different things, but the border between very large planets and very small stars is surprisingly hazy. Astronomers have spotted about 2,000 so-called “brown dwarfs,” objects that are dozens of times larger than Jupiter, but not quite large enough to kick off a sustained fusion reaction. Now, we’re discovered one that could teach us about the dawn of the universe, and it’s right in our cosmic backyard.
Astronomers first became aware of WISEA J153429.75-104303.3 about three years ago, but the details have only started appearing in print. It evaded discovery because its properties don’t match the thousands of brown dwarfs already known to exist. That’s why astronomers have nicknamed it “The Accident.” Following its discovery by citizen scientist Dan Caselden, teams from numerous observatories began zeroing in on The Accident. Since these failed stars can’t sustain fusion, they’re much cooler than main sequence stars like the sun. However, The Accident is even cooler than a brown dwarf should be.
Early in the investigation, scientists at the Keck Observatory in Hawaii scanned The Accident in infrared wavelengths, which is the primary emission window for brown dwarfs. They found almost nothing, suggesting that The Accident is very cold, and therefore very old. Initial observations showed it to be just 50 light-years away from Earth, and the low emissions could have been explained if it was actually much more distant. To verify, NASA turned the Hubble and Spitzer space telescopes toward the anomalous object, confirming that it was in our galactic neighborhood. They also realized The Accident is moving at an incredible speed, roughly half a million miles per hour (800,000 kph).
Photo: NASA/JPL-Caltech
All the evidence pointed to The Accident being ancient, so scientists began looking at how that could affect its composition. When the Milky Way formed 13.6 billion years ago, it was mostly hydrogen and helium. Over time, stars fused those elements into heavier ones like carbon. That’s notable because brown dwarfs are usually rich in methane (which contains carbon). However, the absorbance spectrum of The Accident shows it is low in methane. That makes sense: it formed billions of years ago when the galaxy was carbon-poor.
While it’s not surprising to find such an old brown dwarf, it is surprising to find it so close to home. By analyzing The Accident, scientists hope they’ll be able to spot more objects like it. Its nearness could be a lucky break, or it could point to a larger population of similar objects. These failed stars could tell us a lot about conditions in the universe in the distant past, like super-fast, enormous time capsules just waiting to be unlocked.
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